Light is scattered when it is irradiated on a substance. During the scattering, the wavelength of a large amount of the scattered light does not change, which is described as Rayleigh scattering, and the wavelength of a small portion of the scattered light increases or decreases, which is described as Raman scattering. A spectroscopy to which the Raman scattering corresponds is called Raman spectroscopy. Raman spectroscopy is a spectroscopy of the molecule vibration. Each substance has its own characterized Raman spectroscopy, so Raman spectroscopy can be taken as a “fingerprint” for identifying substances.
The ability of the Raman spectroscopy for identifying substances depends on the resolution of a Raman spectroscopy analyzer. The resolution relates to the focal length, the line density of the gratings and the slit width of the spectrum analyzer, etc. Generally speaking, it is necessary to use a Raman spectroscopy analyzer with a long focal length which is sufficient to increase the resolution. Then, the volume of the whole set of the analyzer system is certainly increased so that it loses portability. In addition, some optic elements in the Raman spectroscopy analyzer are expensive and they have considerably harsh demands for clearness, temperature, humidity of environment, etc., which limits the applications of Raman spectroscopy analyzers in fields like security inspection, environment inspection, chemical analysis, pharmaceutical tests, food tests, etc.